A selector linkage of this kind is described in U.S. Pat. No. 4,723,642, and is also adapted to operate a gear shift means and a line selection means so as to manoeuvre shift wheels (sliding pinions) in the gearbox. Also provided are: force threshold detecting means for detecting the thresholds of forces exerted on the selector linkage; an analog position detector, which is connected to the gear shift means and which is responsive to gear shift information applied to the gear change control lever, the latter being arranged at the "upstream" end of the selector linkage; and a processing and control circuit for, firstly, activating the automatic actuating system for the clutch, and secondly for processing data received by the said force threshold detecting means and by the analog position detector.
In the above mentioned U.S. patent specification, it is possible to detect whether a matched or an unmatched speed is engaged. This state of parity is representative of the position of the gear shift means, the speeds varying according to which of its limit positions is occupied by the gear shift means, such a device comprising an element which constitutes a fixed cam on a gear shift selector shaft and operating, in the limit positions, two feelers which control the operation of electrical contacts connected to the processing circuit. In a variant, an analog position detector that includes a potentiometer is used.
Although such an arrangement indicates whether the speed engaged is matched or unmatched, it does not indicate which speed, that is to say the level at which the line selection means are set.
It is also known, from the periodical "Automotive Engineering" for October 1984, to provide a device which indicates what gear ratio is engaged, by means of a detector for detecting the demultiplication ratio of the gearbox, the device comprising at least two sensors for rotational speed, one of which is responsive to the rotational velocity of the gearbox output shaft, while the other is responsive to the rotational velocity of the gearbox input shaft.
In order to fit sensors in such a device, modifications of the gearbox casing are necessary, and this involves additional machining operations. In addition, such a device will only indicate what gear ratio is engaged at a given moment if the gears are actually rotating, since the information is received on rotating elements.
It is however essential, if automatic operation of the clutch is to be soundly based, that the actual gear ratio engaged be known at any instant, whether the gears are in rotation or not. Indeed, a speed may be engaged when the vehicle is stationary; and it is accepted practice, for example, to give a warning indication to the driver if he tries to start off in third gear.